The present invention relates generally to the field of integrated circuit fabrication. More specifically, the present invention is structure and a technique for fabricating a structure useful for aligning a wafer, substrate, or integrated circuit die.
There are many circumstances where aligning a wafer or integrated circuit die is important. For example, alignment is important during the processing of integrated circuits. Integrated circuits are fabricated using a layer-by-layer process. Alignment may be critical during the processing of certain layers, where these layers must to aligned properly in relationship to previous layers. For example, a contact layer must be aligned properly with a first conductor layer in order to create contacts at the correct locations. Contacts formed at the wrong locations may create shorts or opens, and reduce yield.
Proper alignment is also critical in many other situations. These include testing of an integrated circuit die on a wafer. The die must be positioned properly in order to properly probe the device.
Moreover, alignment is also important when programming or configuring an integrated circuit. For example, a die may have fuses that are to be laser programmed. The fuses may be made of polysilicon, or another conductor. This fuse may couple together two (or more) devices or conductors. A laser with sufficient energy is directed at the fuse. The laser "blows" the fuse, decoupling the two devices or conductors. In order for the laser to properly reference the coordinates of a fuse, the integrated circuit must be aligned properly. When the integrated circuit is not aligned properly, the laser may damage or destroy a portion of the circuitry instead of blowing the desired fuse. Therefore, the alignment of an integrated circuit or wafer is especially critical.
A wafer or integrated circuit die may need to be aligned properly in an X-direction, Y-direction, rotation angle, and other orientation. As technology improves, alignment is becoming more important and critical, especially resulting from the continued scaling and shrinking of semiconductor device geometries. Integrated circuits (or "chips") have progressively become smaller and denser, and any small misalignment will adversely affect the integrated circuit yield and functionality of the integrated circuit.
One technique of alignment is by use of an alignment target. An alignment target should have good reflective or optical contrast so it can be easily identifiable. For example, a laser may be used to determine a change in reflectivity or optical contrast in a semiconductor structure used as the alignment target. This contrast may be achieved by forming a region with rough topography and a region with smooth topography, where these regions are in close proximity to one another. Light is reflected from the smooth or planar region while light is scattered from the rough region. A laser alignment system would find this alignment target and align the wafer or integrated circuit based on the target.
In order to facilitate the fabrication of smaller device sizes, current process technologies emphasize relatively smooth or planar topographies. Smooth topographies allow better step coverage and allow packing of devices and geometries closer together. Some of the techniques used to form flat topographies include such processing techniques as chemical-mechanical polishing (CMP). With CMP, process layers (e.g., dielectrics) are mechanically polished with a slurry mixture to form a very flat topography. A further technique includes plug technology (e.g., tungsten plug technology), where a plug material is used to fill contacts or vias, or both, in order to minimize step coverage (i.e., metal step cover over plugs may be near 100 percent). Plug technology further emphasizes a flat or planar topography.
Despite the substantial success of such planarized process technologies, these processes also meet with certain limitations, especially when used to create a region having good reflectivity, which may be used as an alignment target. With flat or smooth topographies, the resulting structures and regions will have a similar optical reflectiveness. This leads to poor reflective or optical contrast, making it difficult (and possibly impossible) to align a wafer or integrated circuit die, especially by using laser. Furthermore, in some processes, metals use antireflective coatings, further reducing the reflectivity contrast over flat topography.
As can be seen, a structure and technique for fabricating a good reflective contrast is needed, especially where this structure is useful as an alignment target for aligning a wafer or integrated circuit.